It was only a matter of time before BBC4 green-lit a Friday night documentary about southern rock. It is irresistible to connoisseurs of once-unfashionable strains of 70s pop culture, and James Maycock's film Sweet Home Alabama more than does it justice.

Sure, Gregg Allman talks a little slow after his liver transplant, and some of the other "longhaired rednecks" interviewed hardly bring scintillating insight to the topic. But Sweet Home Alabama pulls us back to the early 70s peaks of the Allman Brothers Band and Lynyrd Skynyrd, making us reflect anew on what southern rock really meant.

Was Skynyrd's anthem of the same name a song of defiant pride, cocking a snook at Neil Young's Southern Man, or was it something much worse – a strutting defence of old Confederate values, complete with egregious tip of the stetson to segregationist governor George Wallace? Sweet Home Alabama is a stonking song, but Skynyrd's singer Ronnie van Zant wanted it both ways: to be both a bourbon-chugging rock rebel and the Yankee-baiting bigot that Young was decrying.

"Those of us who have characterised [Van Zant] as a misunderstood liberal," wrote Mark Kemp – one of Maycock's interviewees – in his excellent book Dixie Lullaby, "have done so only to placate our own irrational feelings of shame for responding to the passion in his music."

At least the Allman Brothers had an African American – drummer Jai Johnny "Jaimoe" Johanson – in their ranks. Jaimoe had toured with Otis Redding, arguably the key influence on southern rockers from the Allmans to the Black Crowes, and it was Redding's former manager Phil Walden who, in 1969, set up the label most identified with southern rock – Macon, Georgia's Capricorn Records.

"To the young white southerner, black music always appealed more than white pop music," Walden, who died in 2006, told me. "Certainly the Beach Boys' surfing stuff never would have hacked it in the south. It was too white and it just wasn't relevant. The waves weren't too high down here."

Sweet Home Alabama doesn't shirk from the fact that southern rock was born partly of the deepening racial divide that opened up after Martin Luther King's assassination in 1968. "By the end of the decade, a lot of the results of the civil rights era had served to urbanise black music," Walden said in my 1985 interview with him. "A lot of the people we had considered friends were suddenly calling us blue-eyed devils."

The racial cross-pollination of the southern soul era in Alabama hotspot Muscle Shoals (namechecked in Skynyrd's Sweet Home) came to a shuddering halt. Black music got blacker while white southern rock went back to its first principles of melding country music with rhythm'n'blues.

"In a sense the evolution of southern rock was a reactionary attempt to return rock'n'roll to its native soil," suggested the Texan writer Joe Nick Patoski. "After the decline of interest in rockabilly, white rock in the South had taken a back seat to country and western and soul."

Not that anyone anticipated the way southern rock effortlessly flowed into the post-60s counterculture, with the Allmans eventually co-headlining 1973's colossal Watkins Glen festival with the Band and the Grateful Dead. Along with Skynyrd, who were managed by Phil Walden's brother Alan and whose epic Free Bird mourned the death of Duane Allman, a second wave of southern groups – from the Marshall Tucker Band to Black Oak Arkansas – was soon sweeping the US. Some of them even played a modest part in getting peanut-farming Georgia boy Jimmy Carter into the White House.

Carter, of course, was a liberal and 180 degrees from the segregationist politics of Wallace. So indeed were most of the bands that recorded for Capricorn until the label went bust in the late 70s. Yet the supposed "romance" of the south touted by those outfits is hard to separate from the legacy of slavery and racism.

Southern rock has lived on in the very different iterations represented by the Black Crowes, the Georgia Satellites, the Kentucky Headhunters, Kings of Leon, Drive-By Truckers, American Idol contestant Bo Bice, and of course REM (whose Mike Mills reminisces in Sweet Home Alabama about attending Capricorn's annual picnics). The music's ornery fuck-you spirit meanwhile endures in the work of the charming Toby Keith and his kind. Yet the ambiguities of Van Zant's famous lyric are as troubling as ever, despite the apologia for it offered in Maycock's film by self-styled "redneck negress" Kandia Crazy Horse.

White skin, red necks, blue collars, black music: Sweet Home Alabama tells a quintessential American story that never quite ends.